Salmonella spp. are widespread with in the environment. Their primary habitat is the intestinal tracts of birds, reptiles, animals (especially those on the farm), humans, and occasionally insects. They may also be found in other parts of the body from time to time, for example in the spleen, liver, bile, mesenteric and portal lymph nodes, diaphragm, and pillar in slaughterhouse pigs. Jay, J. M., Modern Food Microbiology, 6th ed. Aspen Publishers, Gaithersburg, Md. (2000).
Serovars that cause human salmonellosis are most often found in foods of animal origin, such as pork and poultry meats, and dairy products. Oosterom, J., Int. J. Food Microbiol. 12:41-52 (1991). The persistence of salmonellae in slaughterhouses and meat processing facilities continues due to the exposure of livestock to environmental sources of contamination, contaminated feeds, and parental transmission of infection. The feces of infected humans and animals contaminate water sources, which subsequently infect farm animals, then contaminate meat during slaughter, and subsequently infect humans, beginning the cycle anew. This cycle is augmented by the practice of international shipping of animal products and feed, which has lead to the worldwide distribution of salmonellosis.
In the 1980s, surveillance data of cattle and human isolates indicate that Salmonella enterica serovar Typhimurium DT104 emerged worldwide. S. Typhimurium DT104 typically is resistant to the antibiotics ampicillin, chloramphenicol, streptomycin, sulphonamides and teracycline (R-type ACSSuT). Threlfall, E. J. et al., Vet. Rec. 134:577 (1994). Currently, data suggest that a multi-resistant Salmonella enterica serovar Newport is emerging in the United States. S. Newport typically is resistant to at least nine antibiotics. Recent studies revealed that 3.5% of retail ground beef was positive for Salmonella spp. of which 35.6% was S. Typhimurium DT104. Zhao T. et al., J. Food Prot. 65:403-407 (2002). Between January and April 2002, a five-state outbreak of S. Newport occurred. Exposure to raw or undercooked ground beef was implicated as the vehicle of transmission. Cattle are thought to be a primary reservoir through which both these multi-resistant pathogens can enter the food supply.
Clinical symptoms of S. Typhimurium DT104 in humans include diarrhea, fever headache, nausea, vomiting and abdominal pain. One-fourth of patients infected in a case-control study had bloody diarrhea, 41% of patients required hospitalization, and 3% of patients died. This is much higher than the case-fatality rate associated for non-typhoid Salmonella infections, which other than for DT104, is approximately 0.1%. Aldina, J. E. et al., J. Am. Vet. Med. Assoc. 214:790-798 (1999).
Surveys of feedlot cattle in the United States done in 1998 revealed that 38% of feedlots were Salmonella spp. positive, and 5.5% of all fecal samples collected were positive for Salmonella spp. S. Typhimurium DT104 was detected in 2.6% of the feedlots, and 2.9% of the positive fecal samples. Fedorka-Cray, P. J. et al., J. Food Prot. 61:525-530 (1998). A similar study conducted on beef cattle in 2000 revealed that 11.2% of all operations tested positive for Salmonella spp., and 1.4% of all fecal samples were positive. Dargatz, D. A. et al., J. Food Prot. 63:1648-1653 (2000). There are clear associations between S. Typhimurium DT104 infection in food production animals and humans. Davis, A. et al., Communicable Disease Report CDR Rev. 6:159-162 (1996).